题干
Helping others is a habit, _________ you can learn even at an early age.
A：which
B：that
C：what
D：when
答案(点此获取答案解析）
A
Helping others is a habit, _________ you can learn even at an early age.
A：which
B：that
C：what
D：when
(α为参数)，在极坐标系中，C2的方程为ρ(3cosθ－4sinθ)＝6，则C1与C2的交点个数为